Organic compounds can be oxidized in many ways including simple combustion.
One well know method for dealing with organic compounds in a waste water stream is known as Wet Air Oxidation (WAO) or the Zimmerman process (U.S. Pat. No. 2,665,249). According to this process, an organic material and an oxidizing agent, frequently air or pure oxygen, are heated in a pressurized reactor so that the reaction temperature remains below the critical temperature of water (about 374.degree. C.) and the pressure is in the range of about 1500 to 2500 psia. At these temperatures and pressures both a liquid and a gas phase are present. Residence times of 0.5 to 1.0 hours result in oxidation of 70% to 95% of the organic compounds in the waste stream.
If more compete oxidation is desired because of the nature of the organic compounds (toxic waste) or for power generation purposes the oxidation may be carried out at critical conditions, typically at a temperature greater than 374.degree. C. and a pressure greater than 3200 psia,
Water at sub-critical temperatures is a poor solvent for non-polar materials (including many organic materials) and a good solvent for polar materials (including many inorganic materials). However, at and above the critical temperature of water many organic compounds become readily soluble in water and many inorganic compounds become insoluble. For example, the solubility of salts in critical water varies from about 1 ppb to about 100 ppm at temperatures from about 450.degree. C. to 500.degree. C. (U.S. Pat. No. 4,338,199 (Modell), column 3, lines 37-41. Oxygen also becomes very soluble in water at temperatures above the critical temperature and the result is that the oxidation of organic compounds takes place very quickly (in seconds). However, the insolubility of inorganic compounds results in a serious scaling problem, which is the problem to which this patent is directed.
U.S. Pat. No. 2,944,396 to Barton and Zimmerman et al. is the first description of the use of critical water oxidation for the oxidation of organic compounds. It describes an improvement to the wet air oxidation process of Zimmerman wherein a second oxidation stage is added. The effluent vapours from the wet air oxidation process are oxidized in a second reactor under critical conditions--842 to 1034.degree. F. (column 5, lines 40-53). The result is substantially complete combustion of all organics (column 5, line 60).
More recently, critical water oxidation processes have been disclosed which directly treat the feed material without a prior wet air oxidation step. U.S. Pat. No. 4,292,953 to Dickinson discloses the critical oxidation of a carboniferous fuel to produce thermal, mechanical or electrical energy. Dickinson notes that if the salt concentration is too high, it can result in scaling of the reactor or scaling or plugging in down stream heat exchange equipment (column 6, lines 33-47).
Modell and U.S. Pat. No. 4,543,190 (Modell No. 2) disclose the use of critical water oxidation to produce useful energy and as a means of desalinating sea water and brine. The organic material may be a waste and/or toxic material or other organic material which is useful as a fuel. The high temperature oxidation produces high pressure steam which may be used for heating purposes. In an alternate embodiment, the water which is fed to the critical oxidizer may be sea water or brine and accordingly contains sodium chloride. According to the disclosure of these patents, the salt precipitates out of the single fluid phase immediately after the oxidation reaction occurs, as in conventional precipitating equipment, thus enabling desalinating of the water in a rapid and continuous process (Modell, column 2, lines 58-63). The patents note that the inorganic material may tend to build upon the walls of conventional apparatus causing hot spots with subsequent destruction of the walls, Accordingly, when the temperature in the oxidizer exceeds 450.degree. C., the disclosure states that the inner wall of the reactor may be coated with a corrosion resistant alloy such as Hastelloy C or, if the reactor has a large diameter, the inner wall may be lined with firebrick (Modell, column 8, lines 14-34).
The scaling problem resulting from the insolubility of inorganic compounds at critical conditions has been a major impediment to the commercialization of critical water oxidation. The process is still not in commercial use.
Different approaches have been developed to handle the scaling problem. U.S. Pat. No. 4,822,497 (Hong et al.) discloses a method of conducting critical water oxidation wherein the reactor has a critical temperature zone in the upper region of the reactor and a lower temperature zone in the lower region of the reactor which has a liquid phase. The critical oxidation occurs in the upper region. Precipitates and other solids from the oxidized critical temperature zone are transferred to the lower temperature zone so as to produce a solution or slurry. The solution or slurry is then removed from the reactor. U.S. Pat. No. 5,100,560 (luang) discloses an alternate method for dealing with the scaling problem in the reactor. According to this disclosure, the reactor once again has a critical temperature zone and a lower temperature zone. At least a portion of the inner wall of the pressure vessel bounding the critical temperature zone is scraped so as to dislodge at least a substantial portion of any solids which may be deposited thereon.
Accordingly, while critical water oxidation could be very useful in various areas of industry, no solution has yet been developed to the scaling problem which arises from the insolubility of inorganic materials in the critical region. Barton et al discloses a process wherein no inorganic materials are present. Hong et al. discloses operating a reactor to have both a critical zone and a subcritical zone. The maintenance of both zones in a single reactor is technically difficult and Hong et al does not contain any example to show that both zones could be maintained in an operating reactor. Huang discloses scraping the inner surface of the critical zone of the reactor of Hong et al to prevent scale build-up. However, Huang discloses that scale build-up does occur and the mechanical action disclosed in Huang would be difficult to operate in practice and would result in a decreased lifetime of the reactor.